1. Technical Field
The present disclosure relates to a printer that can suppress variation in the tension of recording media.
2. Related Art
Inkjet and other types of printers print on a recording medium by driving a printhead synchronized to conveyance of the recording medium continuously or intermittently. When deviation in the conveyance speed of the recording medium occurs, deviation also occurs in the positioning of the printed dots, and good print quality cannot be maintained.
For example, if the back tension applied to the recording medium varies while printing with an inkjet line printer, the amount of slipping (slippage) between the recording medium and the conveyance roller that conveys the recording medium will vary. When slippage changes, the conveyance speed of the conveyance roller—and hence conveyance speed of the recording medium—varies. Because the inkjet head ejects ink droplets and forms a printed image at a regular interval or according to the conveyance distance of the recording medium synchronized to the continuous conveyance of the recording medium, variation also occurs in the printed image due to the change in speed when variation occurs in the conveyance speed of the recording medium. As a result, print quality is adversely affected. When printing on a recording medium with high rotational inertia, such as roll paper, a heavy acceleration load is applied to the recording medium immediately after acceleration, which can greatly affect the print quality. Constructions for absorbing or buffering variation in the back tension on the recording medium are therefore used, as described in JP-A-H08-133540 and JP-A-H08-113403.
The label printer disclosed in JP-A-H08-133540 uses a loop sensor to create slack (a loop) in the roll paper between the roll and the conveyance unit, and the roll paper is conveyed by the conveyance unit with constant tension. If the drive roller is stopped when a loop is desirably formed and the loop sensor then detects that the loop has gotten smaller, the drive roller is driven to advance the roll paper and increase the size of the loop.
The paper feed device described in JP-A-H08-113403 forms a loop in the paper between a loop roller and a guide roller, and the paper is conveyed at a constant speed by a conveyance belt after passing the guide roller. The loop in the paper is detected by a loop detection means (dancer roller). If the loop increases and the loop detection position reaches a minimum level, the paper feed roller that conveys the paper is slowed to increase the loop; if the loop decreases and the loop detection position reaches a maximum level, the paper feed roller is switched to a faster speed to decrease the loop.
The mechanisms according to the related art that buffer or suppress variation in the recording medium tension set a threshold value for detecting slack (loop size) in the recording medium, and either turn the recording medium conveyance operation on/off or switch the recording medium conveyance speed to a high or low speed based on the threshold. The tension on the recording medium can vary greatly when control changes with this type of switching control because the lever of the loop detection means that contacts the recording medium can swing greatly. Significant tension or significant slack can also be produced in the recording medium just before control changes. As a result, the conveyance speed of the recording medium can fluctuate greatly and maintaining good print quality can be difficult.